I think that the reason that no one has offered an answer to your question for the last 20 hours is, in part, because the question is unclear, but i am going to take a guess: It could be that this is what you mean:
Or maybe this:
These are reasonable guesses based on the stated desire to use "function output". However, it may be that you want to replace the constant with a calculation? (This would not be considered a "function") In that case, you can simply put the calculation in the constant block, and define any variables used in the calculation in the Matlab workspace before you run the model.
If you give more information in your question you will get better help. For example, state the actual function calculation you want to use in place of the constant, and it will be more clear.